Videocommunication equipment, such as videoconferencing systems and videophone devices, have enabled people to communicate visually without having to travel to a common location. As a result, communication participants can be separated by large distances.
A typical videoconferencing application uses a video camera to capture a series of images of a target, such as a meeting participant or a document. The series of images is encoded as a data strewn and transmitted over a communications channel to a remote location. For example, the data stream may be transmitted over a phone line, an integrated services digital network (ISDN) line, or the Internet. The encoding process is typically implemented using a digital video coder/decoder (codec), which divides the images into blocks and compresses the blocks according to a video compression standard, such as the ITU-T H.263 and H.261 recommendations. In standards of this type, a block may be compressed independent of the previous image or as a difference between the block and part of the previous image.
In a typical videoconferencing system, the data stream is received at a remote location, where it is decoded into a series of images, which may be viewed at the remote location. Depending on the equipment used, this process typically occurs at a rate of one to thirty frames per second.
In some videoconferencing applications, it is desirable to transmit a high quality still image. Until the image is completely received and decoded, the receiving terminal is often unaware of its content. Some decoders decode and display a block only after they have received the complete image. With the image being transmitted as a series of blocks, considerable delay is often involved in transmitting the entire image. For example, in applications where the available bandwidth for transmitting data is small, transmission of a 352.times.288 pixel image may require up to a minute. In order to transmit still images more quickly, a lower image quality may be used for compressing the image.